How do I turn off the unread message count on the Windows XP login screen?

Open notepad and create file disable_unread_msg_display.reg with the following content:

---
REGEDIT4

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\UnreadMail]
"MessageExpiryDays"=dword:00000000
---

Now import it by double-clicking it in Windows Explorer or by running it from Command Line. This file will create zero REG_DWORD value "MessageExpiryDays" under the UnreadMail key and will effectively cause all messages to be naturally "expired", thus not displayed...